Instant messaging or IM is a real-time, usually text-based system where messages are sent to other users on the system. This can be one-to-one or one-to-many. One-to-many systems are often called...
Depends on the email provider. But, for a simple answer, if you are old enough to read, write, and type, you are old enough to have an IM account.
But some require you to be at least 13 years old.